Liquid toner dispersions for electrophotography are generally prepared by dispersing pigments or dyes, and natural or synthetic resins in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
Images are photoelectrically formed on a photoconductive layer mounted on a conductive base. The layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing an image over the surface with a laser or L.E.D. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light, with an electrostatic charge remaining in the image area. Charged pigment and/or dye particles from the liquid toner solution contact and adhere to the image areas of the plate. The image is then transferred to the desired substrate such as a carrier sheet.
In some electrophotographic processes, a photoconductive member is charged to a uniform potential and then exposed to light in a manner that forms a latent electrostatic image. This latent image is developed by immersing it in a liquid toner. A development electrode is often used to aid in this process. In the image area, the electrostatic charge is retained by the photoconductor and in the non-image areas the charge is completely or nearly completely dissipated. The development electrode is set at an electrical potential such that the toner particles will experience a relatively small force moving them away from the photoconductive member in the non-image area. One such case would be a selenium photoconductor that has received a positive charge and been exposed such that the non-image area has a potential of ca. 150 volts whereas the image area retains the majority of its initial potential which might be on the order of 1000 volts. The development electrode would then be set at ca. 200 volts so that the electric field would be in the direction of the photoconductor in the non-image area and in the opposite direction in the image area. A toner whose particles bore a net negative charge would then be attracted to the image area and slightly repelled from the non-image area.
One problem in such systems is that the toner usually contains color imparting pigment particles that bear both signs and even some that are uncharged. Among the problems that are created by this heterogeneity are that the positively charged particles will deposit in the background areas, leaving marks that are not intended. Further the uncharged particles will not respond to either electrical field although they may respond to the areas of nonuniform field by dielectrophoresis, and, as such, will be deposited in the liquid carrier creating a general fog over the entire image. The heterogeneity may also adversely affect the stability of the toner as the negatively charged particles will tend to form strong agglomerates (flocks) with the positively charged particles. When prints are made, the particles will, of course, be removed from a working liquid toner dispersion in proportion to their charge and so the liquid toner both will tend, as the printing run continues to increasingly accumulate particles that bear either the "wrong" sign, a small proportion of "correct" sign or no sign at all. This will adversely affect the performance of the developer. One readily apparent manifestation of this problem is an increasing reduction in the average net "correct" charge on the toner particles.
The above problems are particularly troublesome in high speed electrophotographic printing processes wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like traveling at speeds of greater than 100 ft./min. These high speed processes contrast to the normal office-type photocopier situation wherein, in most instances, time between successive image formation is substantially longer. Stated differently, in high speed operations, the electrophoretic velocity of the charge color imparting particles is dependent upon their charge. This reduction in electrophoretic velocity decreases the speed at which successful development can occur, thus hampering optimum high speed printing performance.